The proposed research seeks to discover what children and adults know about differences in thinking. More specifically, it investigates whether and under what conditions subjects of different ages understand that different people (or even the same person on two occasions) will have different trains of thought. The three proposed studies address the following questions respectively: 1) Will children and adults recognize that differences in thinking will occur even when two very similar people see the same stimulus and construe it the same way?; 2) If told that differences in thinking are occurring, will they be able to adequately explain the differences?; and 3) Will performance differ if participants are asked to make judgments not about the thoughts of two others, but about their own thoughts and those of another?. It is hypothesized that the ability to infer and explain these differences requires knowledge about the way thinking works that young children might not possess. These studies will test this hypothesis and provide insight into children's developing knowledge about differences in people's mental lives. If children do indeed lack an understanding of the ways in which people's mental lives differ, they may have a difficult time interacting with and understanding those around them. By shedding light on children's theories about such differences, this research may point to ways in which this understanding could be increased leading to cognitive, social, and emotional benefits for children.